As the aircraft climbed to 1,350ft the crew engaged the autopilot, but the turboprop responded by pitching down and rapidly descending.

Investigators determined that the autopilot had been attempting to capture a target altitude of zero – effectively meaning that the aircraft had inadvertently been instructed to dive to ground level.

The aircraft’s systems issued sink warnings and a “pull up” order to the crew, and the captain recovered the descent at 928ft after disconnecting the autopilot, says the UK Air Accidents Investigation Branch.

Analysis showed the aircraft lost 500ft in around 18s and reached descent rates of 4,300ft/min.

It found that the aircraft’s flight-director modes had not been set correctly before the Q400’s departure – pointing out that the crew was under time pressure – and the inquiry turned up three other incidents, all occurring last year, involving similar incorrect settings on Flybe Q400s.

Two of these other three events also involved pilots’ rushing to expedite departure or mitigate delays, the investigation states.

Revision of a loadsheet had threatened a delay and the captain, the flying pilot, had chosen to complete several tasks before pushback, including setting the flight-director modes.

He selected the ‘go-around’, ‘heading’ and ‘altitude select’ modes but did not enter a target departure clearance altitude.

The inquiry states that, unbeknown to the crew, the flight-director sequence entered by the captain resulted in mode changes, including a default switch to ‘altitude’ mode, which were not detected by the pilots.

Flybe’s training regime mentions the importance of setting the target altitude before the flight-director modes, warning that the system could otherwise capture the aircraft’s current altitude – which would be ground level.

Although the captain, during line-up, noticed that ‘altitude select’ was not displayed, choosing this mode when ‘altitude’ is also selected does not change the target altitude.

The target altitude should have been 3,000ft but the system had already captured a ground-level setting.

As the aircraft’s autopilot was engaged during the climb, it attempted to reach this incorrect target altitude, putting the turboprop into a dive.

“The [captain] reacted promptly in accordance with the trained sequence of actions and returned the aircraft to a safe flightpath,” says the inquiry.

Flybe has introduced a check to its flight-data monitoring scheme which will look for instances of incorrect flight-director mode selection before departure, it adds.

None of the 48 occupants of the Q400 (G-ECOE) was injured during the incident, which took place on 11 January this year.